Approaches to the wireless power transfer field include use of induction such as an inductive charging pad, where an induction coupling takes place between the pad and the device to be charged. This approach, however, does not effectively reduce the need for the wires that transfer the power except for the last couple of inches between the recipient of power (receive site) and the patch that is hardwired to the power sources (outlet).
Another approach for wireless power transfer recommends using the power transmitted to the far field of a transmit antenna. However, the efficiency of this approach is very low since transmitted power decays proportional to the square of the distance from the transmit antenna to the receive site.
Another approach for wireless power transfer uses a generated near-field in transferring electric power from one resonant antenna to another in a system of two closely coupled antennas. This approach, however, requires antennas too large to fit into most portable devices in the receiving end of the power transfer system such as a laptop, a PDA, or a wireless handset. For example, in one wireless power transfer experiment between a transmit antenna and a receive antenna similarly shaped with a radius of 300 mm and a height of 200 mm and about 2 meters apart, the energy transfer efficiency was around 40 percent. The efficiency is further reduced when one of the antennas is substantially smaller than the other. The efficiency due to such size mismatch results in a decrease in energy transfer efficiency significantly below 10 percent.
Furthermore, location of the receive antenna over an electrically shielded or ferrite-based region also results in reduced efficiencies. Specifically, ferrite materials are lossy materials resulting in lower efficiencies for wireless power transfer. Accordingly, there is a need for a receive antenna and devices attached thereto that extend the efficiency of power transfer to a smaller dimensioned receive antenna.